The method of X-ray diffraction with isomorphic substitution has been extended to aqueous solutions containing complex ions. In a study of isomorphism between NO₃^-(aq), BrO₃^-(aq) and ClO₃^-(aq), it is found that the NO₃^- and BrO₃^- ions are isomorphic up to the level of the first-order difference indicating a similarity in their anion-hydration structure. Based on the isomorphism of these two ions, and because the water molecules are only weakly perturbed from their structure in pure water, a second-order difference calculation was made and the ion-ion pair correlation function of these two ions obtained. The results show that the most probable ion-ion separation is between 5 AA and 11 AA and the distance of closest approach is 4.8 AA. NO₃^- and BrO₃^- are found, however, not to be isomorphic with ClO₃^-.